Google search for sp flash tool. V5.164 is latest one I think. You also need the usb driver for mt6755 devices for the sp flash tool to work.Then it's quite easy to do. From flash tools select the Mt6755 scatter file, switch off the phone, select firmware upgrade, from the drop down menu. Click on download, then plug in the phone. Takes about 45 sec to flash.

Warning though, this factory reset your phone. All data on will be deleted, so make sure to back up.